Do you own the bus? That may have an effect on what is considered a school facility.
571.030.
1. A person commits the crime of unlawful use of weapons if he or she knowingly:
(1) Carries concealed upon or about his or her person a knife, a firearm, a blackjack or any other weapon readily capable of lethal use; or ...
(10) Carries a firearm, whether loaded or unloaded, or any other weapon readily capable of lethal use into any school, onto any school bus, or onto the premises of any function or activity sponsored or sanctioned by school officials or the district school board.
4. Subdivisions (1), (8), and (10) of subsection 1 of this section shall not apply to any person who has a valid concealed carry endorsement issued pursuant to sections 571.101 to 571.121 or a valid permit or endorsement to carry concealed firearms issued by another state or political subdivision of another state.
571.107. ... No driver's license or nondriver's license containing a concealed carry endorsement issued pursuant to sections 571.101 to 571.121 or a concealed carry endorsement or permit issued by another state or political subdivision of another state shall authorize any person to carry concealed firearms into: ...
(9) Any place where the carrying of a firearm is prohibited by federal law; ...
(10) Any higher education institution or elementary or secondary school facility without the consent of the governing body of the higher education institution or a school official or the district school board. Possession of a firearm in a vehicle on the premises of any higher education institution or elementary or secondary school facility shall not be a criminal offense so long as the firearm is not removed from the vehicle or brandished while the vehicle is on the premises;
Safest bet, legally, is to contact the AG. Also, read any contracts you have with the school system or employee handbooks. Make damn sure you can secure the firearm in case you need to leave the bus unattended.